http://docs.mongodb.org/manual/tutorial/expand-replica-set/

To include information from:

http://docs.mongodb.org/manual/administration/replica-sets/#adding-members

And then revise the following blurb to point to the revised first link:

http://www.mongodb.org/display/DOCS/Upgrading+to+Replica+Sets#UpgradingtoReplicaSets-UpgradingFromReplicaPairsorMaster%2FSlave

A section reads:

Now there are two paths we can take: either resyncing the slaves from scratch or using their existing data. Resyncing takes longer. Using the existing data is only possible if the slave was up-to-date before the replica pair was shut down and you add it to the replica set before the master has handled "too many" new writes (the size of the oplog determines what "too many" is).

Followed by the heading: Resyncing the Slaves

However there is never a definition of how to deal with turning a slave into a replicaset member with existing data.